Mybatis使用 - Mybatis xml复杂映射 - 《深入学习 Java 系列》

复杂映射指的是使用xml来进行一对一，一对多和多对多的开发测试，其实要理解的是多对多的本质还是一对多，在数据库中的体现要借用中间表。
涉及到的表有四个：
一对一为订单表和用户表
一对多为用户表和订单表
多对多为用户表、权限表以及中间表
表的具体内容查看码云。
具体代码如下：
// 涉及到的实体表对应对象：不展示getset和tostring
package com.wangzhi.pojo;
public class Orders {
    private Integer id;
    private String name;
    private Integer userId;
    private String createTime;
    private User user;
}
package com.wangzhi.pojo;
import java.util.List;
public class User {
    private Integer id;
    private String name;
    private List<Orders> ordersList;
    private List<Role> roles;
}
public class Role {
    private Integer id;
    private String rolename;
}
// mapper接口
public interface OrdersMapper {
    Orders selectOrderById(Integer id);
    List<Orders> selectAll();
}
public interface UserMapper {
    User selectById(Integer id);
    List<User> selectAll();
    List<User> selectUserRole();
}
// xml
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.wangzhi.mapper.OrdersMapper">
    <!--一对一的map-->
    <resultMap id="id_user" type="orders">
        <result column="id" property="id" javaType="int"/>
        <result column="name" property="name" javaType="string"/>
        <result column="userId" property="userId" javaType="int"/>
        <result column="createTime" property="createTime" javaType="string"/>
        <association property="user" javaType="com.wangzhi.pojo.User">
            <result column="userId1" property="id" javaType="int"/>
            <result column="userName" property="name" javaType="string"/>
        </association>
    </resultMap>
    <sql id="ALL_COLUMN">
        id, name, user_id, create_time
    </sql>
    <!--resultMap: 手动配置实体属性和表字段的映射关系，存在多个实体的对应关系的时候使用-->
    <select id="selectOrderById" resultMap="id_user" parameterType="int">
        SELECT
            orders.id AS id,
            orders.name AS name,
            orders.user_id AS userId,
            orders.create_time AS createTime,
            user.id AS userId1,
            user.name AS userName
        FROM orders INNER JOIN user ON orders.user_id = user.id
        WHERE orders.id = #{id}
    </select>
    <select id="selectAll" resultMap="id_user" parameterType="int">
        SELECT
        orders.id AS id,
        orders.name AS name,
        orders.user_id AS userId,
        orders.create_time AS createTime,
        user.id AS userId1,
        user.name AS userName
        FROM orders INNER JOIN user ON orders.user_id = user.id
    </select>
</mapper>
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.wangzhi.mapper.UserMapper">
    <resultMap id="userMap" type="user">
        <result column="userId" property="id" javaType="int"/>
        <result column="userName" property="name" javaType="string"/>
        <!--一对多-->
        <collection property="ordersList" ofType="com.wangzhi.pojo.Orders">
            <result column="orderId" property="id" javaType="int"/>
            <result column="orderName" property="name" javaType="string"/>
            <result column="orderUserId" property="userId" javaType="int"/>
            <result column="orderCreateTime" property="createTime" javaType="string"/>
        </collection>
    </resultMap>
    <select id="selectAll" resultMap="userMap">
        SELECT
            user.id AS userId,
            user.name AS userName,
            orders.id AS orderId,
            orders.name AS orderName,
            orders.user_id AS orderUserId,
            orders.create_time AS orderCreateTime
        FROM user LEFT JOIN orders ON user.id = orders.user_id
    </select>
    <select id="selectById" resultMap="userMap" parameterType="int">
        SELECT
        user.id AS userId,
        user.name AS userName,
        orders.id AS orderId,
        orders.name AS orderName,
        orders.user_id AS orderUserId,
        orders.create_time AS orderCreateTime
        FROM user LEFT JOIN orders ON user.id = orders.user_id WHERE user.id = #{id}
    </select>
    <resultMap id="userRoleMap" type="user">
        <result column="userId" property="id" javaType="int"/>
        <result column="userName" property="name" javaType="string"/>
        <!--多对多，本质其实还是一对多-->
        <collection property="roles" ofType="role">
            <result column="roleId" property="id" javaType="int"/>
            <result column="roleName" property="rolename" javaType="string"/>
        </collection>
    </resultMap>
    <select id="selectUserRole" resultMap="userRoleMap">
        SELECT
                user.id AS userId,
                user.name AS userName,
                sys_role.id AS roleId,
                sys_role.rolename AS roleName
        FROM
                user LEFT JOIN sys_user_role ON USER.id = sys_user_role.userid
                     LEFT JOIN sys_role ON sys_user_role.roleid = sys_role.id
    </select>
</mapper>
// 测试类
public class TestOrderMapper {
    @Test
    public void testOneToOne() throws IOException {
        InputStream resourceAsStream = Resources.getResourceAsStream("SqlMapperConfig.xml");
        SqlSession sqlSession = new SqlSessionFactoryBuilder().build(resourceAsStream).openSession();
        OrdersMapper mapper = sqlSession.getMapper(OrdersMapper.class);
        Orders orders = mapper.selectOrderById(1);
        System.out.println(orders);
    }
    @Test
    public void testOrdersAll() throws IOException {
        InputStream resourceAsStream = Resources.getResourceAsStream("SqlMapperConfig.xml");
        SqlSession sqlSession = new SqlSessionFactoryBuilder().build(resourceAsStream).openSession();
        OrdersMapper mapper = sqlSession.getMapper(OrdersMapper.class);
        List<Orders> orders = mapper.selectAll();
        orders.forEach(System.out::println);
    }
}
public class TestUserMapper {
    @Test
    public void testOneToMany() throws IOException {
        InputStream resourceAsStream = Resources.getResourceAsStream("SqlMapperConfig.xml");
        SqlSession sqlSession = new SqlSessionFactoryBuilder().build(resourceAsStream).openSession();
        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        List<User> users = mapper.selectAll();
        users.forEach(System.out::println);
    }
    @Test
    public void testManyToMany() throws IOException {
        InputStream resourceAsStream = Resources.getResourceAsStream("SqlMapperConfig.xml");
        SqlSession sqlSession = new SqlSessionFactoryBuilder().build(resourceAsStream).openSession();
        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        List<User> users = mapper.selectUserRole();
        users.forEach(System.out::println);
    }
}
多对多的展示是看以哪方为主，两方都可以为主。重点是resultMap的使用以及collection和association的使用。